In an attempt to understand cardiac metabolism on a molecular level it is proposed to study the complexes between lactate dehydrogenase from pig heart and reduced nicotinamide adenine dinucleotide (NADH). Specifically the role of strain of the C-H bond on the "A" side at the number four position of the nicotinamide ring of NADH will be evaluated in its complexes with LDH in an effort to determine whether the bond that will be broken in the overall reaction is perturbed in any of the enzyme-substrate complexes before the reaction has occurred. If deuterium is substituted for the hydrogen subtending the bond in question, perturbation of the bond will require more energy than that required for perturbation of the analogous bond in nonisotopic NADH. Therefore, the enzyme will bind less of the deuterated coenzyme than the nonisotopic coenzyme, and measurement of the isotope effect on the binding constant for NADH should determine whether strain is significant. The role of strain will be evaluated in the binary complex (LDH-NADH) and in the ternary complexes (LDH-NADH-lactate and LDH-NADH-oxamate) by incubation of the enzyme with a mixture of deuterated and nondeuterated coenzyme (plus the second ligand for the ternary complexes) and separation of the bound from the unbound coenzyme by ultrafiltration. If the ratio of the two forms of coenzyme is different in the bound and unbound fractions, an isotope effect is demonstrated.